A hearing aid is typically worn by someone who suffers from hearing loss, and can compensate for the hearing impairment by amplifying the local sound field. Hearing aids operate in either a microphone (acoustic) mode or a telecoil (inductive) mode. In the microphone mode, sound waves that are incident upon a microphone which is integrated in the hearing aid are converted into an electrical audio signal. In the telecoil mode, an induction coil (also referred to as a telecoil or T-coil) which may also be inside the hearing aid picks up the local magnetic field that has been modulated by the receiver or a dedicated coil of a nearby telephone handset. In both modes, the resultant electrical audio signal that has been picked up is subsequently processed, amplified and then converted to sound (by a small speaker inside the hearing aid) that can be heard by the user.
Hearing aids do not always function well with some portable communication devices such as mobile phones. One problem experienced by those who wear hearing aids and have a mobile phone is that the microphone inside the hearing aid may pickup unwanted ambient acoustic noise from the surrounding background environment, in addition to the desired speech coming from the mobile phone receiver. This makes it difficult for the user to discern the desired speech. However, when the hearing aid is switched to its T-coil mode, the hearing aid microphone may be deactivated, and the T-coil is inductively coupled, via the local magnetic field, to the voice coil of the receiver in the mobile phone. As such, environmental or background acoustic noise is not amplified by the hearing aid, when the T-coil is being used as a pickup. Hearing aid compatible (HAC) mobile phones are becoming more commonly available to the public. In addition to the typical acoustic receiver, HAC phones may also include a separate magnetic field radiator, such as a loop of wire, also referred to as a telecoil or T-coil, specifically designed for inductively coupling with the T-coil of a nearby hearing aid. Such phones are thus compatible with both the microphone of a hearing aid, as well as its T-coil. These mobile phones may include a switch that enables a user to manually select a HAC mode of operation. In that mode of operation, the audio signal processing that is applied to a desired audio signal is modified to change the frequency response of the audio signal processing chain, so as to better accommodate the microphone of a hearing aid. Another change that may be made when the HAC mode has been selected is to allow the desired audio signal that is being applied to drive the voice coil of the receiver to also drive the telecoil that is inside the mobile phone.
A few mobile phones contain an active noise cancellation (ANC) block that is typically implemented using digital audio signal processing techniques, to help reduce or cancel out the acoustic background noise that may be heard by a user of the mobile phone. The ANC block attempts to cancel out the acoustic background noise by producing what is referred to as an anti-noise signal. The anti-noise signal is combined with the desired audio content, and then the receiver voice coil is driven with this combined signal. The goal is that the receiver will produce an anti-noise acoustic signal that should, in theory, cancel the acoustic background noise that otherwise would be heard by the user. This technology is also used in noise-canceling headphones.